Comments on Spin-Lattice Relaxation

Microscopic interpretations of an experiment measuring the temperature dependence of spin-lattice relaxation in rare-earth salts are discussed with the aid of a systematic general formalism. The discussion clarifies the relation between "two-step" phonon relaxation processes and resonant phonon-scattering processes, both of which have been proposed in the literature, to explain the experimental results. We show that a complete treatment of the two-step process---that is, a process involving two consecutive single-phonon direct processes---includes the results previously ascribed to resonant scattering processes and, in addition, gives small corrections to the reported results.